1. Field of the Invention
The present invention relates to a solar power generating device and, in particular, to a solar power generating device for tracking the operating point of a solar cell at the maximum point thereof.
2. Description of the Related Art
The output voltage-output current characteristic of the solar cell used in the solar power generating device are expressed by the curved line shown in FIG. 4A. Accordingly, the output voltage-output power characteristic of the solar cell are expressed by the curved line shown in FIG. 4 B. Namely, while the output voltage of the solar cell is between 0 (V) and a predetermined voltage, the output power steadily increases. However, once the output voltage of the solar cell exceeds the predetermined voltage the output power steadily decreases. The output power at the above-described predetermined voltage is taken as the maximum power of the solar cell, and the operating point of the solar cell at the above-described predetermined voltage is called the maximum power point P.sub.M of the solar cell.
As a control for producing the maximum power from a solar cell having this type of characteristic, a maximum power point tracking control (hereinafter, referred to as MPPT control) is known in which the operating point of the solar cell constantly tracks the maximum power point P.sub.M.
This MPPT control causes a designated voltage value which becomes the solar cell operating voltage target value to change slightly at predetermined intervals and compares the output power from the solar cell measured at this time with the value measured at the previous time. By changing the above-described designated voltage value in the direction in which the output power is constantly increasing, the MPPT control can bring the solar cell operating point close to the maximum output point (the optimum operating point).
Conventionally, this type of MPPT control was carried out so that, in order for the solar cell operating point to reach the maximum power point in a short time when the solar cell was started, each of the virtual optimum operating voltage, the MPPT minimum voltage V.sub.L, and the MPPT maximum voltage V.sub.H (depending on the type of solar cell being used) were set as fixed values and the output power from the solar cell in the range only between the MPPT minimum voltage V.sub.L and the MPPT maximum voltage V.sub.H was set as the maximum.
However, the solar cell output voltage-output power characteristic is determined not only by the type of solar cell, but also varies according to the amount of solar radiation and changes in the temperature around the periphery of the cell which accompany seasonal and other variations. Namely, as is shown in FIG. 5, the output voltage-output power characteristic changes in the direction in which the optimum operating voltage decreases as the solar cell peripheral temperature increases. Moreover, the output voltage-output power characteristic changes in the direction in which the optimum operating voltage increases when the amount of solar radiation increases.
However, in the aforementioned conventional MPPT control, because the virtual optimum operating voltage, the MPPT minimum voltage V.sub.L, and the MPPT maximum voltage V.sub.H (depending on the type of solar cell being used) were set as fixed values, there were instances when the actual optimum operating voltage was not inside the range between the MPPT minimum voltage V.sub.L and the MPPT maximum voltage V.sub.H, which had been set as fixed values, because of factors such as the temperature around the periphery of the solar cell. This led to the problem that the power generated by the solar cell was not able to be used effectively.
Further, the output voltage-output power characteristic differs depending on the total surface area of the solar cell. Generally, when a solar power generating device is installed, a plurality of solar cell panels are connected in series so as to obtain a predetermined output power. However, depending on conditions such as the size of the location where the panels are installed and the environment around that location, the number of solar cell panels which can actually be installed sometimes differ, leading to the output voltage-output power characteristic to also differ greatly. Therefore, the problem exists that, conventionally, the generated power of an actually installed solar cell can not be effectively used because data such as the virtual optimum operating voltage are set in advance as fixed values.